A transfer sheet having a pressure-responsive adhesive layer offers the advantage of good workability in that it can be easily applied to a substrate surface but has several disadvantages. Thus, because of the very nature of a pressure-responsive adhesive agent, the bond strength that can be attained is limited; offset tends to take place after application to the substrate surface, re-application is difficult, and air is liable to be entrapped and the entrapped air can hardly be driven out.
On the other hand, a transfer sheet having a regular adhesive layer insures a permanent bond but in order that a sufficient bond strength may be insured, subsequent drying and heating are essential. Moreover, in application to the substrate surface, a pressure must at any rate be applied for a time period which is not brief. Therefore, the sheet is not satisfactory in workability.
The foregoing suggests that if a stick-adhesive composition that would undergo spontaneous transition from a pressure-responsive adhesive to an adhesive after application to any substrate should ever be developed, both the demand for ease of application and the demand for permanent adhesion could be successfully reconciled.
Much research has been undertaken into stick-adhesive agents. By way of illustration, Japanese Kokai Patent Application No. 63-10680 discloses a stick-adhesive composition comprising a monofunctional acrylic monomer, a compound containing two or more acryloyl groups, a compound containing two or more epoxy groups and a masked epoxy curing agent.
Japanese Kokai Patent Application No. 51-6235 discloses a stick-adhesive composition comprising a mixture of n-butyl acrylate and 2-ethylhexyl acrylate, a functional vinyl monomer and a vinyl monomer copolymerizable therewith.
Furthermore, Japanese Kokai Patent Application No. 61-66631 describes a continuous lapping method for cladding a piece of furniture or an architectural member with a decorative sheet employing a urethane prepolymer stick-adhesive composition which comprises paying out an endless decorative sheet from a roll, applying said stick-adhesive composition to the reverse side of said decorative sheet in the course of its travel, drying the same and applying the sheet to the furniture or architectural member through a roll assembly adapted to compress the sheet against said member in register with the surface configuration of the latter. The stick-adhesive composition employed in this prior art method develops a pressure-sticking power necessary for application within 8 hours after evaporation of the coating solvent and, thereafter, manifests an adhesive property as the curing reaction proceeds in line with progressive absorption of moisture with or without the aid of a curing component.
However, the stick-adhesive composition proposed in Japanese Kokai Patent Application No. 63-10680 has the disadvantage that heating is required for conversion from a sticking agent to a bonding agent. The stick-adhesive composition described in Japanese Kokai Patent Application No. 51-6235 is also disadvantageous in that thermal curing is required after application to the substrate. This need for a post-heating procedure for said conversion from a sticking agent to an adhesive agent is not only an economic deterrent, because this requires additional work after application, but a serious limiting factor in the utility of the composition because it cannot be utilized in applications where heat is abhorred or in the field of transfer printing sheets which are to remain on the substrates.
The stick-adhesive composition according to Japanese Kokai Patent Application No. 61-66631 is subject to the time limit that its pressure adhesive property is available only up to 8 hours after evaporation of the coating solvent. Moreover, it can never be used for transfer sheets which are marketed in the condition preformed with a stick-adhesive layer.
It is, therefore, an object of the present invention to provide a transfer sheet with which a final product can be marketed as carrying a sticker layer and applied to a substrate surface by simple pressing as it is the case with the conventional pressure-responsive adhesive and which permits re-application for correction, does not allow entrapment of air and, after application, insures a permanent bond after a certain time period.